The invention relates to a process for the preparation of 1,2-dichloroethane that is very pure with respect to chloral or/and chloral hydrate and carbon dioxide, which process comprises oxychlorination of ethylene, using hydrogen chloride and an oxygen-containing gas such as air or oxygen, and alkali treatment of the dichloroethane, and also to 1,2-dichloroethane prepared in accordance with that process.
A known process for the preparation of 1,2-dichloroethane is the oxychlorination of ethylene using hydrogen chloride and oxygen, wherein chloral or/and chloral hydrate are formed as undesirable by-products. Also, the product of the process often comprises relatively large amounts of dissolved carbon dioxide formed as a by-product of the reaction of ethylene with oxygen.
The publication DE 1 518 931 describes an oxychlorination process for the preparation of 1,2-dichloroethane, wherein the undesirable chloral is removed by means of a condensing step. However, only 75 to 80% of the chloral can be separated out from the product by means of that process step. The remaining chloral is subsequently converted, by increasing the pH, into substances, for example chloroform, that can be more readily separated out by means of distillation.
Similarly, German Patent Specification 1 468 480 describes an oxychlorination process wherein the undesirable by-products chloral and/or chloral hydrate are converted into sodium formate and chloroform by means of alkali treatment and can therefore be readily separated out from the product of the process, 1,2-dichloroethane.
The disadvantage of both processes is that a large quantity of the alkali solution, especially aqueous alkali metal hydroxide solution, is consumed in neutralising the carbon dioxide and a corresponding amount of alkali metal carbonate or other salts is formed. Larger amounts of alkali solution therefore have to be used than would be necessary purely for converting the chloral and/or chloral hydrate. In addition, the salt charge formed in considerable measure as a result of the neutralisation has to removed from the production plant and disposed of.
The problem of the present invention accordingly was to provide, for the preparation of 1,2-dichloroethane, an oxychlorination process that avoids the disadvantages of the known processes. The problem of the present invention was especially to reduce, or to avoid altogether, the consumption of alkali solution, and the salt charge, during conversion of the undesirable by-products chloral and chloral hydrate and, at the same time, to prepare 1,2-dichloroethane that has only a low content of the said by-products, if any at all.